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BQ24725A: BQ24725A BATDRV issue

sp lee
Prodigy 110 points
Part Number: BQ24725A

Normally when the system is powered by the battery, the BATDRV turns on the N-CH FET. In the application circuit the system voltage is also connected to the AC power through another FET driven by ACFET. So my question is if BQ24725A guarantees that the BATDRV turns off the battery FET during charging. I know if somehow the AC power adapter doesn't meet those strict requirement, the battery FET will be ON, which potentially connect the AC power adapter directly to the battery.

  • 0
    TI__Genius 11850 points

    Hi,

    Yes. When the adapter is not present, BATDRV will be high at SRN + 6V in order to turn on the BATFET and power the system from the battery. When the adapter is present and charging the battery, ACFET/RBFET will turn on, and the BATFET will turn off. The BQ24725A has break-before-make logic to prevent shoot-through current when switching between adapter and battery power for the system load. Section 8.4.3 in the datasheet provides more details about this.

    The BQ24725A is a traditional topology charger, so you can take a look at Figure 3 in this TI blog post, which shows the states of the ACFET, RBFET, and BATFET in both adapter mode and battery mode:

    https://e2e.ti.com/blogs_/archives/b/fullycharged/archive/2016/09/19/understanding-battery-charger-features-and-charging-topologies

    Best regards,

    Angelo

  • 0 in reply to Angelo Zhang87
    Prodigy 110 points

    Hi Angelo,

    Thanks for your reply. I'm just afraid either "break before make" or "make before break" will result in a system power glitch when the power adapter is plugged in during normal system operation. So I'm thinking to use a simple schottky diode to replace the BATFET. Besides the lower efficiency, is there any other disadvantage of this approach?

  • 0 in reply to sp lee
    TI__Genius 11850 points

    Hi,

    The break-before-make logic ensures that there will not be shoot-through current when the system power switches between the adapter and the battery. The BQ24725A is designed to handle this, as described in the System Power Selection section (8.4.3) of the datasheet. Are you concerned that the system voltage will have a brief dip during the transition from battery power to adapter power?

    It is possible to use a Schottky diode in place of the BATFET, but this causes additional power loss, as you pointed out. Keep in mind that the BATFET itself has a body diode oriented from BAT to SYS, so I'm not sure I understand why you would like to use a Schottky diode instead of the BATFET.

    Best regards,

    Angelo

  • 0 in reply to Angelo Zhang87
    Prodigy 110 points

    Hi Angelo,

    What I worried about is that the BATFET is ON at the wrong moment because of timing issues, so that the power adapter voltage applies to the battery directly. That's why I would like to use schottky diode. Anyway, thanks for your help.

  • 0 in reply to sp lee
    TI__Genius 11850 points

    Hi,

    Okay, thanks for clarifying. The system power selector ensures that the BATFET will not be on at the same time as the ACFET and RBFET, so you do not need to worry about a direct short from the adapter to the battery if you use the BATFET.

    Best regards,

    Angelo